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Johne's disease

Johne's disease

Johne’s disease is a common cause of death and reduced growth rates among deer. It is also known as paratuberculosis and there are two main strains in New Zealand. Deer are mostly affected by the cattle strain. The disease is caused by a bacterial infection (Mycobacterium avium subspecies paratuberculosis).The infection starts in the small intestine and spreads to the lymph nodes. If the disease advances the intestinal walls can thicken, interfering with digestion and absorption of nutrients.

How does the disease spread?Johne’s disease is most often introduced onto an uninfected farm through newly purchased deer.

On farm, infection occurs through deer eating pasture or drinking water contaminated with contaminated faecal material. Hinds can also pass the bacteria on to their fawns through the placenta, colostrum or milk.

What are the symptoms?Clinical signs are muscle wasting, ill thrift and diarrhoea. Stress can bring on clinical effects (for example during lactation, the rut, or during drought). Sometimes enlarged lymph nodes or cheese-like abscesses can be seen. Typically infected deer show only subclinical symptoms (slow growth rates) rather than obvious symptoms. This means it can be hard to recognise the problem.

Effect on deer productionJohne’s disease outbreaks typically affects young deer aged 8-15 months, with losses of up to 20% recorded. Sporadic cases in mixed age deer occur in 1%-3% per annum.

DiagnosisThere are several diagnostic tests. The Paralisa, ELISA, Gel Diffusion or CFT tests involve taking a blood sample. These are best used for confirming Johne’s in a deer with clinical signs of the disease. The Paralisa test is considered to be the most sensitive but it is still not 100% accurate. This is because other bacteria can cause lesions. A negative test result indicates a low chance of infection but does not guarantee freedom from infection.

Faecal samples can be cultured but it takes up to 60 days. As shedding of the bacteria in the faeces is intermittent, not all infected animals (especially those with minor infections) will show up as positive. Culture is the best to identify different strains.

If the animal is going to slaughter, Johne’s can be confirmed by culturing intestine and lymph nodes. Often the first indication of Johne’s in a herd is from the processing plant. Enlarged lymph nodes at slaughter are noted by meat inspectors and the number of animals with lesions reported back to farmers. Farmers can use this information to help put a control programme into place.

Once introduced, Johne's disease can’t be eradicated. Control requires a combination of measures including

culling affected animals

vaccination

testing to detect deer with subclinical symptoms (for culling)

Minimising cross-infection between age groups.

using older animals - which are more resistant - to clean up contaminated pastures.

avoiding the grazing of weaners on areas contaminated by clinically affected deer, cattle or sheep.

vaccination

Vaccination is an option for finishing animals destined for slaughter. A single dose at weaning is required and it reduces the incidence of clinical disease among the herd (it does not prevent infection altogether). All vaccinated animals must be ear-marked and the slaughter plant notified. It must not be given to breeding animals because it is very likely to interfere with TB testing and give false positives.

Culling decisionsWhen a high number show clinical effects consider blood sampling and doing the Paralisa test on all animals. Any positive animals should be culled.

The Paralisa test can identify high (>100) and low (<100) titre levels. Low titre deer may be kept as they may have some resistance and be less likely to shed the bacteria. They may be retested later and culled if titres have increased. High titre level deer should be culled immediately.

Professional adviceIn 2007, Deer Industry New Zealand established Johne's Management Limited to keep farmers informed about the disease and help advise on the most effective methods of control. Check out what it can do for you online.If Johne's disease is suspected then seek advice from a veterinarian in the Johne's Consultant Network. Click here to find a Johnes consultant in your regionAn animal health plan, created in consultation with a veterinarian specialising in Johne’s disease control, will help manage the disease.

Information on internal parasites is available in a convenient DINZ Deer Fact sheet (August 2015). Print off your own copy here >>